unchanging

A NetHackintrinsic that can be acquired only by wearing or eating an amulet of unchanging (no other methods are known at this time). If you have unchanging, polymorph effects (such as the potion, the wand, the ring, traps, or anything else) will not cause you to change form. If you are currently polymorphed, you will not change back to your original form after a time interval, as is usual.

Note that this can get you into trouble if you aren't careful. Ordinarily, if you are fatally wounded while polymorphed into something else, you revert back to your original form with full hit points instead. If you have unchanging, you'll just die immediately. This can be surprising and of course extremely unfortunate.